Spark-integrated propellant injector head with flashback barrier

ABSTRACT

High performance propellants flow through specialized mechanical hardware that allows for effective and safe thermal decomposition and/or combustion of the propellants. By integrating a sintered metal component between a propellant feed source and the combustion chamber, an effective and reliable fuel injector head may be implemented. Additionally the fuel injector head design integrates a spark ignition mechanism that withstands extremely hot running conditions without noticeable spark mechanism degradation.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was supported in part by subcontract number 1265181 fromthe California Institute of Technology Jet Propulsion Laboratory/NASA.The U.S. Government may have certain rights in the invention.

This application claims priority to “Injector Head”, Ser. No.60/868,523, filed Dec. 4, 2006.

BACKGROUND

Liquid fueled rockets have better specific impulse (I_(sp)) than solidrockets and are capable of being throttled, shut down and restarted. Theprimary performance advantage of liquid propellants is the oxidizer. Theart of chemical rocket propulsion makes use of controlled release ofchemically reacted or un-reacted fluids to achieve thrust in a desireddirection. The thrust acts to change a body's linear or angularmomentum. There are multiple methods for using liquid propellants toachieve thrust.

A monopropellant is a single fluid that serves as both a fuel and anoxidizer. Upon ignition of a monopropellant, a chemical reaction willoccur yielding a mixture of hot gases. The ignition of a monopropellantcan be induced with use of an appropriate catalyst, introduction of ahigh energy spark, or raising a localized volume beyond the reaction'sactivation energy. Monopropellant ignition causes an exothermic chemicalreaction whereby the monopropellant is converted into hot exhaustproducts. A common example of a monopropellant is hydrazine, often usedin spacecraft attitude control jets. Another example is HAN (hydroxylammonium nitrate). Another form of propellant is a bipropellant, whichconsists of two substances: a fuel and an oxidizer. Bipropellants arecommonly used in liquid-propellant rocket engines. There are manyexamples of bipropellants, including RP-1 (a kerosene-containingmixture) and liquid oxygen (used in the Atlas rocket family) and liquidhydrogen and liquid oxygen (used in the Space Shuttle).

Chemically reacting monopropellants and pre-mixed bipropellants liberatechemical energy through thermal decomposition and/or combustion. Thischemical energy release is initiated by a mechanism deposed within thecombustion chamber (i.e., the chamber where a majority of chemicalenergy release occurs). Commonly, the initiation mechanism isincorporated in the vicinity of a combustion chamber's fuel injectorhead. The design and manufacture of a fuel injector head used in acombustion chamber is important to achieve effective and safe operationof the rocket thruster. If the design is not correct, flame canpropagate back past the fuel injector head and into the propellantstorage tank (known as flashback) causing a catastrophic system failure(i.e., an explosion).

SUMMARY

Implementations described and claimed herein address the foregoingissues with a fuel injector head that incorporates specific designcriteria that allows it to be used effectively with monopropellants ormixed bipropellants. The fuel injector head provides thorough mixing ofpropellant fuel and oxidizers prior to injection into a combustionchamber. Furthermore, the fuel injector head provides a flame barrier toprevent flames or combustion waves from back-propagating into thepropellant feed system including sustained combustion processes. Inaddition, the fuel injector head provides a novel configuration thatintegrates a regenerative fluid-cooled spark igniter into the rocketthruster assembly so as to protect the spark igniter (i.e., theelectrode) from degradation due to the high temperatures from propellantcombustion in the combustion chamber. The unique and novel fuel injectorhead design disclosed herein provides a substantial improvement in theart of rocket thrust technology, allowing use of a wide array ofpropellants for rocket propulsion. Moreover, similar to fuel injectorheads and propellants that have found application in other gasgeneration, combustion processing, and power generation applications,the present technology may be utilized in these types of applications aswell.

Certain implementations of the technology provide a combustion systemcomprising: a housing defining a cooling chamber and a combustionchamber separated by a flame barrier, wherein the cooling chamber isdisposed around an electrode assembly, the flame barrier comprises fluidpaths with a diameter of less than about 10 microns, and the electrodeassembly comprises an interface sheath encompassing an insulating tubewhich encompasses an electrode; and a fuel inlet tube is disposedthrough the housing into the cooling chamber.

In yet other implementations, a combustion system is providedcomprising: a housing defining a chamber having distal and proximalends; the housing defining a cooling chamber at the proximal end, acombustion chamber at the distal end and a flame barrier between thecooling chamber and the combustion chamber; an electrode assemblydisposed through the proximal end of the housing through the coolingchamber and through the flame barrier terminating at a surface of theflame barrier adjacent the combustion chamber, wherein the electrodeassembly comprises an electrode disposed within an insulating tube, andwherein the insulating tube is disposed within an interface sheath; anda fuel inlet tube disposed through a side of the housing into thecooling chamber.

In some aspects of these implementations, the combustion systemcomprises a flash barrier having fluid paths having a diameter of lessthan about 10 microns, or less than about 7 microns, or less than about5 microns, or less than about 1 micron, or less than about 0.5 micron,or less than about 0.2 micron, or less than about 0.1 micron. In yetother aspects, such as those associated with atmospheric and lowpressure applications, the flame barrier comprises fluid paths having adiameter of less than about 2 cm, or less than about 1.5 cm, or lessthan about 1 cm, or less than about 0.5 cm, or less than about 0.25 cm,or less than about 0.1 cm.

In yet other aspects, a combustion system is provided, wherein theinterface sheath and the flame barrier comprise materials having similarcoefficients of thermal expansion. In some aspects, the combustionsystem is provided wherein the interface sheath and the flame barriercomprise stainless steel alloys, pure nickel, nickel alloys, niobium,rhenium, molybdenum, tungsten, tantalum, tantalum alloys, sinteredceramic or laminate structures. In other aspects, the combustion chambercomprises an ablative or high temperature liner adjacent the housing,and in some aspects, the combustion chamber defines a throatconstriction at the distal end of the housing.

In certain aspects of the combustion system, the electrode comprises atip, single point, double point, triple point, quadruple point, star orsplit configuration. Also in some aspects, the combustion system furthercomprises a seal between the flash barrier, the cooling chamber and thehousing. In aspects of the combustion system, the cooling chamberreceives fuel via the inlet tube.

Yet other implementations of the technology provide a method forpreventing flashback between a combustion chamber and a feed propellantand for providing regenerative cooling of an electrode assemblycomprising: providing a propellant inlet into a cooling chamber, whereinthe cooling chamber circumscribes the electrode assembly; providing amicro-fluidic flame barrier to separate the cooling chamber and acombustion chamber, wherein the micro-fluidic flame barrier comprisesfluid paths having a diameter of about 5 microns or less; and runningfeed propellant through the fuel inlet, into the cooling chamber andthrough the flame barrier.

In some aspects of this implementation, the flame barrier comprisesfluid paths having a diameter of about 10 microns or less, or about 7microns or less, or about 5 microns or less, or about 2 microns or less,or about 1 micron or less, or about 0.5 micron or less, or about 0.2micron or less or about 0.1 micron or less. In yet other aspects, suchas those associated with atmospheric and low pressure applications, theflame barrier comprises fluid paths having a diameter of less than about1 cm.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a longitudinal cross sectional view of a fuel injector headaccording to the claimed invention.

FIG. 2 is a cross sectional view of the fuel injector head as seen frominside a combustion chamber.

FIG. 3 illustrates the effective quenching distance for one exemplarycombustible N₂O and fuel gas mixture versus the mixed propellantdensity. In this case quenching distance is estimated experimentallyfrom the media grade particle size above which the filter will not pass.

FIG. 4 is an illustration of geometry and parameters useful forunderstanding thermal distribution in a flame barrier and pressure dropacross a flame barrier. T_(adiabatic) is the flame temperature; q_(rad),q_(cond), q_(conv) are the radiative, conductive, and convective heatfluxes respectively

FIG. 5 is an illustration of internal flame barrier temperature andpressure drop through an exemplary porous media flame barrier exposed toa chamber heating surface heat flux.

FIG. 6 is an illustration of analysis conducted to determine sensitivityof propellant pressure drop across the flame barrier and flame barriercombustion chamber face temperature as a function of flame-frontposition from the flame barrier face.

FIG. 7 is an illustration of experimental measurements of flame barrierpressure drop versus propellant mass flux.

FIG. 8 is a longitudinal cross sectional view of the disclosed fuelinjector head integrated into a prototype rocket thruster with a hightemperature liner.

FIG. 9 is a cross sectional view of the disclosed fuel injector headintegrated into a sophisticated regeneratively-cooled rocket thruster.

FIG. 10 is an isometric view of a regeneratively-cooled rocket thrusterthat utilizes the disclosed fuel injector head.

FIG. 11 is an illustration of exemplary thermal analysis predicting thepropellant preheat temperatures that a regeneratively-cooled rocketthruster's fuel injector head may encounter.

FIG. 12 is an illustration of pressure drop versus propellant mass flowrates before and after a filter has been subjected to oven heating atthree different temperatures of 500° C., 750° C., and 1000° C.

FIG. 13 is experimental tensile testing data of one sintered media flamebarrier.

FIG. 14 is an illustration of the fuel injector head integrated into amonopropellant rocket engine application undergoing testing andverification.

DETAILED DESCRIPTION

Implementations described and claimed herein address the foregoingissues with a fuel injector head that incorporates specific designcriteria that allows it to be used effectively with monopropellants orpre-mixed bipropellants. In addition, the fuel injector head provides anovel configuration that integrates a regenerative fluid-cooled sparkigniter into the chemical reactor so as to protect the spark igniter(i.e., the electrode) from degradation due to the high temperatures fromcombustion in the combustion chamber. The unique and novel fuel injectorhead design disclosed herein provides a substantial improvement in theart of rocket propulsion allowing for use of a wide array ofpropellants, including those that combust at very high temperatures.Similar to fuel injector head and propellants that have foundapplication in other working fluid production and power generationapplications, the present technology may be utilized in these types ofapplications as well.

Before the present devices and methods are described, it is to beunderstood that the invention is not limited to the particular devicesor methodologies described, as such, devices and methods may, of course,vary. It is also to be understood that the terminology used herein isfor the purpose of describing particular embodiments only, and is notintended to limit the scope of the present invention; the scope shouldbe limited only by the appended claims.

It should be noted that as used herein and in the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “astructure” refers to one structure or more than one structure, andreference to a method of manufacturing includes reference to equivalentsteps and methods known to those skilled in the art, and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. All publications mentioned areincorporated herein by reference for the purpose of describing anddisclosing devices, formulations and methodologies that are described inthe publication and that may be used in connection with the claimedinvention, including U.S. Ser. No. 60/868,523, filed Dec. 4, 2006entitled “Injector Head”, and U.S. Ser. No. 60/986,991, filed Nov. 9,2007 entitled “Nitrous Oxide Fuel Blend Monopropellant.”

Where a range of values is provided, it is understood that eachintervening value, between the upper and lower limit of that range andany other stated or intervening value in that stated range isencompassed within the invention. The upper and lower limits of thesesmaller ranges may independently be included in the smaller ranges andare also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails. In other instances, well-known features and procedures wellknown to those skilled in the art have not been described in order toavoid obscuring the invention.

The art of chemical rocket propulsion makes use of controlled release ofchemically reacted or un-reacted fluids to achieve thrust in a desireddirection. The thrust acts to change a body's (i.e., the rocket's)linear or angular momentum. Similar to fuel injector heads andpropellants that have found application in other working fluidproduction and power generation applications, the claimed invention maybe utilized in many alternative types of applications as well, includinggas generation for inflation systems and inflatable deployments, insystems used to convert thermal energy in hot exhaust gases tomechanical and electrical power, and in high energy storage media forprojectiles, munitions, and explosives. Examples where the claimedtechnology could be applied specifically include earth-orbitingspacecraft and missile propulsion systems; launch vehicle upper stagepropulsion systems and booster stages; deep space probe propulsion andpower systems; deep space spacecraft ascent and earth return stages;precision-controlled spacecraft station-keeping propulsion systems;human-rated reaction control propulsion systems; spacecraft landerdescent propulsion, power, and pneumatic systems for excavation,spacecraft pneumatic science sample acquisition and handling systems;micro-spacecraft high performance propulsion systems; military divertand kill interceptors; high altitude aircraft engines, aircraft backuppower systems; remote low temperature power systems (e.g., arctic powergenerators); combustion powered terrestrial tools including hightemperature welding and cutting torches as well as reloadable chargesfor drive mechanisms (e.g., nail guns, anchor bolt guns), and the like.Moreover, there are many derivative applications related to usingcombustion stored energy and the delivery systems therefor.

In the case of many terrestrial combustion power applications (e.g., gasand diesel engines), the oxidizer is commonly atmospheric air whichconsists of oxygen that is highly reactive in the combustion reactionand relatively inert gases such as nitrogen. Bipropellants are eitherinjected as separate fluids into a chemical reaction chamber or mixedimmediately prior to injection (e.g., in carbureated or fuel-injectedpiston combustion engines).

FIG. 1 is a cross sectional view of various components of a fuelinjector head 100 according to the claimed invention. Such a fuelinjector head would be a component of a rocket thruster assemblyElectrode 102, when sufficiently charged, induces a dielectric breakdownof uncombusted combustion fluids (propellant components). A tip 116 ofelectrode 102 is seen as well. The significance of tip 116 is discussedin detail infra. Surrounding the electrode 102, is a high temperaturedielectric insulating tube 104. The function of the dielectricinsulating tube 104 is to create a dielectric barrier between theelectrode 102 and an interface sheath 106, necessary to control thelocation where a spark propagates between electrode 102 and theinterface sheath 106. The combination of the electrode 102, dielectricbarrier 104, interface sheath 106, electrical connector (not shown) andpower supply (also not shown) comprises the spark ignition assembly. Inaddition, the interface sheath 106 aids in joining the electrode to asintered and/or micro-fluidic flame barrier 108. Additionally, theinterface sheath 106 shields the high voltage spark propagated from theelectrode from inducing electromagnetic interference in other componentsof the rocket thruster. Gas tight interfaces 110 and 112 are createdbetween the electrode 102 and the dielectric insulating tube 104 as wellas between the dielectric insulating tube 104 and the interface sheath106. A preferred implementation utilizes a brazed seal at gas tightinterfaces 110 and 112; however, in some cases, a bonded interface maybe used instead. The sintered and/or micro-fluidic flame barrier 109comprises micro-fluidic passages to provide a fluid-permeable barrierbetween the combustion chamber and incoming combustion reactants. Ajunction 114 between the interface sheath 106 and the sintered and/ormicro-fluidic flame barrier 108 may utilize an interference fit, awelded joint, a brazed joint, or a bonded joint depending on thematerials employed, the nominal operating conditions, and the chemicalreaction (propellant type) for which the fuel injector head is intended.Note the electrode 102, dielectric barrier 104, and interface sheath 106(the “electrode assembly”) of the spark ignition assembly is shown in anexemplary concentric configuration to the injector flame barrier 4. Thisexemplary concentric configuration is not necessarily required to beable to perform any of the functions described or claimed herein, asother configurations may be employed equally effectively.

Materials effective for use for the dielectric insulating tube 104typically are high-temperature dielectric insulating ceramics. In someprototypes that have been tested, alumina was used, but other insulatormaterials also appropriate for the dielectric insulating tube includebut are not limited to boron nitride, magnesium oxide, titanium nitride,titanium oxide, and beryllia. An additional consideration in theselection of materials for the dielectric insulating tube 104 is thethermal conductivity of the tube. Tubes with higher thermal conductivityaid in transferring heat from the electrode to the feed propellantkeeping the electrode cooler (as discussed in detail, infra). Coolerelectrodes tend to have longer service lives.

The interface sheath 106 serves in part to help cancel electromagneticinterference (EMI) generated by the spark ignition assembly and to matewith the sintered and/or micro-fluidic flame barrier 108. High power,pulsed, or high frequency sources can generate electromagnetic noisethat can interfere with nearby electronics. Because electrical sparkignition often requires a high power, pulsed or high frequency current,minimizing the resultant EMI noise generated from this source from otherelectrical components may be desirable. Here, if the signal and returnare constrained to a concentric electrically conductive geometry (e.g.,the configuration of the electrode 102, the dielectric insulating tube104, and the interface sheath 106 as shown in FIGS. 1 and 2), theelectromagnetic noise that would be generated in the vicinity of theinjector head can be significantly reduced. In general, the power supplyfor generating the high voltage pulses and the high voltage lineconnecting the power supply to the electrode 102 will also have theirown similar EMI mitigation measures incorporated into their designs.Additionally, the material from which the interface sheath 106 is mademust typically has a coefficient of thermal expansion (CTE) that issimilar to the material of the sintered and/or micro-fluidic flamebarrier 108.

Stresses at joint 114 induced by heating conditions commonly encounteredin combustion applications may cause joint failure. Alternatively or inaddition, if an interference fit is made with a sintered ormicro-fluidic flame barrier comprising a material with a dissimilar CTE,a small gap may form at joint 114. A joint failure and/or release at 114may lead to flame propagation around the sintered and/or micro-fluidicflame barrier causing the fuel injector head to fail in its intendedpurpose of preventing flame back-propagation back up the propellant feedsystem line to the propellant storage reservoir. This type of failure iscommonly known as flashback and is described in more detail below. Forthis reason, the material used for the interface sheath 106 preferablyeither is the same as the sintered and/or micro-fluidic flame barrier108, or, alternatively, the CTEs of the different materials used forthese two components is closely matched based on the anticipatedtemperatures that the components will have to endure. For fuel injectorheads of the claimed invention, a nickel 200 interface sheath 106 wasused. Other materials that may be employed for the interface sheath 106and the sintered and/or micro-fluidic flame barrier 108 may include, butare not limited to, various stainless steel alloys, pure nickel, variousnickel alloys, niobium, rhenium, molybdenum, tungsten, tantalum, andalloys thereof. For the particular assembly shown, 5 micron media gradenickel 200 was utilized. Other fuel injector heads used with differentpropellants in different applications can utilize different materials.In some implementations, the flash barrier comprises fluid paths havinga diameter of less than about 10 microns, or less than about 7 microns,or less than about 5 microns, or less than about 1 micron, or less thanabout 0.5 micron, or less than about 0.2 micron, or less than about 0.1micron. In yet other aspects, such as those associated with atmosphericand low pressure applications, the flame barrier comprises fluid pathshaving a diameter of less than about 2 cm, or less than about 1.5 cm, orless than about 1 cm, or less than about 0.5 cm, or less than about 0.25cm, or less than about 0.1 cm.

FIG. 2 is a cross sectional view of the fuel injector head as seen fromthe combustion chamber, showing sparker geometry and exemplary sparkassembly placement. The electrode tip geometry and the materialselection of the electrode 200 are important features. A sharp tip 208is created on the electrode 200 on the combustion chamber side of theelectrode 200, which serves to concentrate an electromagnetic field attip 208 (tip 208 may also be seen in a different perspective in FIG. 1at 116). Concentrated electromagnetic fields allow for generation of avoltage breakdown necessary for generating a spark. An arcing spark, ifsufficiently energetic, will ignite a combustible fluid. The gap of thearc is commonly set to allow both minimum voltages to be applied inorder to generate a spark and provide sufficient spark gap energy toinitiate the combustion process. Every gas mixture has a differentvoltage breakdown curve (breakdown voltage versus variable,pd≡mixture_pressure*gap_distance) that is dependent on combustible gaspressure, gap distance, and gap geometry. Therefore, gap distances andapplied voltages to the electrode may vary depending on the combustiblegas mixture and electrode tip geometry. In general, a wide array ofelectrode tip geometries (e.g., single point, double point, triplepoint, quadruple point, star pattern, split electrode, etc.), inaddition to the exemplary tip geometry shown in FIG. 2, will produceelectric fields necessary for generating a spark in a combustiblemixture that is capable of initiating an exothermic combustion process.Also seen in cross section in FIG. 2 are the dielectric insulating tube202, the interface sheath 204, and the sintered and/or micro-fluidicflame barrier 206.

The sintered and/or micro-fluidic flame barrier (seen in FIG. 1 at 108)is designed to prevent flames and/or initial combustion (deflagrationand/or detonation) waves from reaching the uncombusted propellant in apropellant feed system. Typically during ignition, combustions waves aregenerated that must be prevented from interacting with the uncombustedpropellant in the propellant feed system which could cause a flashback.For relatively steady-state flow applications (i.e., rocket engine),after ignition, a relatively steady-state flame-front will form andreside downstream of the flame barrier (FIG. 4). In other processes(e.g., a piston engine) the flame-front may momentarily interact withthe flame barrier at each combustion cycle in which case the flamebarrier also acts as a thermal reservoir to absorb combustion thermalenergy during this short duration interaction and dissipates the thermalenergy into the next cycle's uncombusted inlet propellant duringinjection.

A very important parameter for designing the flame barrier 108 is thequenching distance of a monopropellant. This is the smallest flowpathdimension through which a flashback flame can propagate. In actualpractice this dimension (here, approximately the diameter of amicro-fluidic flowpath) is affected by additional parameters such astortuosity (curviness of flow path) and to a lesser extent thetemperature of the solid containing the flowpath. Some propellants haveflame quenching distances on the order of microns. Smaller flowpathsizes will quench a flame and, in general, prevent flashback. However,secondary ignition by heat transfer through a solid that is in contactwith the unreacted monopropellant must also be ultimately considered(flame barrier thermal analysis is described below).

FIG. 3 illustrates exemplary experimental data of sintered metal poresizes sufficient for quenching a nitrous oxide and fuel bipropellantthat has been mixed at propellant densities associated with 100-1000psia combustible gases and liquid/gas mixtures. At lower propellantdensities such as combustible gas mixtures operating at atmosphericpressure, the quenching distance increases significantly (>mm). Thequenching distance is a function of the propellant density in the pores,which in turn is dependent on the liquid/gas being used, pressure, andinlet temperature. As pore sizes decrease in a flame barrier design, thepressure drop across the filter element will, in general, increase suchthat arbitrarily small pore sizes are not necessarily feasible (pressuredrop analysis is described in more detail below). In the experiment fromwhich this data is derived, a 10 foot×¼ in stainless steel line wasloaded with premixed propellant with the sintered metal flame barrier onone end. The line was intentionally detonated. A combustible solid onthe opposite side of the flame barrier was monitored to determine if aback-propagation through the flame barrier had occurred.

FIG. 4 illustrates flame barrier, flame-front, and propellant fluidparameters and geometry useful for understanding how quasi-steady-statecombustion thermal interactions effect propellant pressure drop andinternal flame barrier temperatures. α and β are viscosity and inertiaflow coefficients, respectively, that are correlated with the flamebarrier filter pore size and micro-fluidic fluid geometry andtortuosity.

During operation, the sintered media and/or micro-fluidic media flamebarrier 108 (also seen at 804 of FIGS. 8 and 900 of FIG. 9) cause(s) afluid pressure drop. This pressure drop needs to be considered in thedesign of an upstream pressurant system. In general, the pressure dropmechanism in the fuel injector head also helps to filter out pressureoscillations associated with combustion instabilities in a combustionand/or chemical reaction chamber (820, 902) that could ultimately leadto catastrophic chamber failure. The fuel injector head is designed toaccommodate a specific flow rate of propellant, differential pressure,and combustion chamber operating pressure. In general, the flow rate ofpropellant and operating pressure are commonly specified for aparticular application. For example, by combining the mass flow ofpropellant and desired combustion chamber operating pressure withknowledge of the combustion chemistry and rocket nozzle design, it ispossible to determine the output thrust a rocket engine will produce. Insuch a scenario, for a desired rocket engine thrust and nominaloperating chamber pressure, the sintered media and/or micro-fluidicflame barrier (108, 804, 900) would be designed to provide a desireddifferential pressure drop for the prescribed mass flow rate ofpropellant. In combination with an upstream feed system pressurantdesign, this differential pressure drop would ensure that the desiredcombustion chamber pressure is achieved and/or maintained duringoperation. To adjust the differential pressure drop, the flame barrierthickness and cross-sectional area to the mass flow can be varied.

The pressure drop gradient ({right arrow over (∇)}P is pressure drop perunit length that fluid traverses through injector medium) across theinjector/flame barrier is related to the rate of propellant mass fluxthat passes through the flame barrier ({right arrow over ({dot over(m)}″_(p) is the propellant mass flow rate per unit surface area), thefluid density of fluid traveling through the flame barrier (ρ), thepropellant's dynamic viscosity (μ), and typically flame barrierfluid-interaction parameters, a and β. An exemplary mathematicalexpression that relates all of these injectorhead and propellant fluidparameters is:

${\overset{\rightharpoonup}{\Delta}\; P} = {{- \frac{{\overset{\overset{.}{\rightharpoonup}}{m}}_{p}^{''}}{\rho}}\left( {\frac{\mu}{\alpha} + \frac{{\overset{.}{m}}_{p}^{''}}{\beta}} \right)}$

In practice, particularly for two-phase (combination liquid and gas)flows, this relationship can be more complicated such that actualexperimental measurements of pressure drop through the flame barrierversus mass flow rate under similar operating conditions as would beencountered in real application is a better technique for ultimatelyderiving flame barrier specifications. It is worth noting that sincepressure drop is dependent on fluid density and temperature, and dynamicviscosity is dependent on temperature, combustion processes will, ingeneral, influence the pressure drop through the flame barrier.

FIG. 5 illustrates an exemplary analysis (based on the pressure droptheory of diffusive flow as described above) of propellant temperatureand pressure as propellant traverses through a porous media flamebarrier with a radiative and convective heat flux on the combustionchamber face of the flame barrier.

FIG. 6 illustrates the sensitivity of propellant fluid pressure dropacross the flame barrier and surface temperature (chamber-side) of theflame barrier as a function of the location of the flame-front. In thiscase, an exemplary propellant with an adiabatic flame temperature(T_(adiabatic) is the maximum combustion temperature of a combustedpropellant) of 3177° C. is analyzed using heat transport andthermophysical properties of the uncombusted and combusted exemplarypropellant.

FIG. 7 illustrates an example measurement of experimental pressure dropacross a fuel injector head flame barrier. As a preliminary step in theinjector head design process, it is often advantageous to define theflow characteristics of a flame barrier. The experiment that generatedthe data shown in FIG. 7 utilized a number of pressure transducers(electrical sensors used to measure fluid pressure) and mass flowmeasurements to determine both propellant mass flow rate and thepressure drop across a flame barrier. Mass flow rate is converted into anormalized mass flux by dividing the mass flow rate by thecross-sectional area of the exposed flame barrier. The resultant curvegenerated from this data can be used to size the cross-sectional area ofa flame barrier for a given mass flow rate and desired differentialpressure drop across the flame barrier, or alternatively can be used toestimate pressure drop for a given flame-barrier design for example.

Typical manufacturing methods for producing small fluid paths in amachined device (e.g., drilling, punching, etc.) for the most part areincapable of or are uneconomical for producing a viable fuel injectorhead to address the small required quenching distances. However, porouscomponents, such as may be created by sintering pre-sorted media, caneffectively create flow paths as small as 0.1 micron and smaller. In oneimplementation, sintered metal is produced by means of a powderedmetallurgy process. The process involves mixing metal powder of aspecific grain size with lubricants or additional alloys. After themixture is complete, the mixed powder is compressed (e.g., an exemplaryrange of pressures is between about 30,000 lbs. and about 60,000 lbs ormore per square inch) by machine to form a “compact”, where typicalcompacting pressures are between 25 and 50 tons per square inch. Eachcompact is then “sintered” or heated in a furnace (e.g., to atemperature lower than the melting point of the base metal) for anextended period of time to be bonded metallurgically. In oneimplementation, the sintered metal contains micro-fluidic passages thatare relatively consistent in composition, providing flow paths as smallas 0.1 micron or less.

One fuel injector head prototype tested utilized a sintered metal filteras the flame barrier between the combustion chamber and the propellantinlet. However, other porous materials having micro-fluidic passages maybe used in alternative designs including sintered ceramic filters andlaminate structures. The fuel injector head design shown in FIG. 1 anddescribed herein facilitates two major functions, namely, creation of aflame proof barrier and integration of a propellant spark-ignitionmechanism. In the case of bipropellants or propellants with multipleconstituents, however, the diffusive barrier can also provide a meansfor mixing propellant constituents very thoroughly prior to injectioninto a combustion or chemical reaction chamber by utilizing a highlytortuous network of micro-fluidic passages.

In general, the combustion process generates very high temperatures. Thegeometries shown in FIG. 8 and FIG. 9 help mitigate electrode heating byutilizing the incoming combustible propellant as a regenerative (i.e.,where thermal energy is not lost) coolant. Nevertheless, radiative,conductive, and convective heating of the electrode in a hightemperature combustion chamber commonly results in temperatures that arehigher than many conventional metals' operating limits. Furthermore,electrode life is generally longer with higher temperature electrodematerials when exposed to high temperature chemical reaction andcombustion processes. Thus, in some implementations, higher temperatureelectrode materials are used such as but not limited to refractorymetals including tungsten, molybdenum, niobium, tantalum, rhenium, andalloys thereof. Niobium has been used effectively in numerous prototypefuel injector head prototypes and was used in the prototypes tested suchas shown in FIG. 11. Niobium possesses a number of favorable attributesincluding a close CTE match with exemplary alumina electrical insulatorswhich helps prevent tensile stresses (common failure mechanism inceramics) from being generated in the interface sheath (seen at 104 inFIG. 1) under high temperature thermal loading, resistance to thermalshock, high ductility and high strength. The ductility is particularlyattractive for fabrication processes that utilize cold working as afundamental fabrication procedure. In one implementation, manufacturingcomprised three primary steps. First, the end of a Niobium rod wasflattened by mechanically deforming the tip. Second, the tip was bent toachieve a 90° bend. Finally, the excess material was removed to create apart dimensionally and geometrically similar to that shown in FIGS. 1and 2. Alternative methods of manufacturing include machining(traditional or (electrical discharge machining), mechanical forming,sinter pressing, molding, casting, punching, welding (by electrode,e-beam or laser), or a combination thereof.

FIG. 8 is a longitudinal cross sectional view of a ceramic-lined rocketthruster to demonstrate an exemplary configuration of the fuel injectorhead as a component of the rocket thruster. In this implementation,combustor reactants enter through a propellant inlet tube 810, enter acooling chamber 826, travel through the sintered and/or micro-fluidicflame barrier 804, ignite within the combustion chamber 820, travelthrough an ablative liner 802, and exit through the thrust throatconstriction 822. Between the propellant inlet tube 810 and the sinteredand/or micro-fluidic flame barrier 804, the un-reacted propellant flowsinto a cooling chamber 826 that provides cooling to the fuel injectorhead (the combination of components comprising electrode 816, dielectricinsulating tube 814, interface sheath 812, and sintered and/ormicro-fluidic flame barrier 804 as described in the detailed descriptionof FIGS. 1 and 2). Recall that a seal is created 824 at the junction ofthe sintered and/or micro-fluidic flame barrier 804 with the thrustercase 800. Seal 824 can be created by welding, brazing, bonding, ormechanical interference. An additional seal 818 is created at thejunction of the thruster body cap 808 to the interface sheath 812.Depending on application and material choice, seal 818 can be made by abraze joint, weld joint, mechanical interference fit, or bonded joint.However, as discussed previously, the use of proper seals is imperativein proper fuel injector head function in many implementations. Improperintegration of the fuel injector head assembly into a rocket thruster(e.g., improper fit or faulty seals) can pose a substantial safety risk.Prototypes built and used tested successfully have utilized acombination press/brazed flame barrier outer seal 824, and a brazedinterface shield/thruster body cap seal 818. Note also in this crosssectional view are the dielectric insulating tube 814 and the electrode816. A BNC (Bayonet Neil-Concelman)-type electrical connector 806 is anexemplary common electrical connector that may be used to interface ahigh voltage line to the electrode 816 and facilitate current deliveryfrom and current return to a high voltage power supply.

Another feature of the fuel injector head of the claimed invention isthe integration of an actively cooled spark ignition mechanism. Some ofthe particular monopropellants for which the integrated fuel injectorhead was created combust at an extremely hot temperature (around 3200°C.). Therefore, placing conventional sparking mechanisms (i.e.,electrodes) in the combustion chamber would result in melting of nearlyany electrode material. However, because the electrode and surroundingdielectric insulating tube and interface sheath are cooled (e.g., byincoming fluid delivered by the propellant inlet tube 810 and coolingchamber 826 of FIG. 8), very hot exothermic combustion reactions may besustained without degrading the sparking mechanism.

FIG. 9 is a cross sectional view of a regenerative cooled rocketthruster truncated slightly below the combustion chamber to demonstrateadditional features. In this implementation, the combustion reactantsencounter the fuel injector head via an annular regenerative coolingpathway 914 which cools the combustion chamber, flame-barrier joint 25,and the electrode assembly portion of the spark ignition assembly. Thecombustion reactants then pass through the sintered and/or micro-fluidicflame barrier 900, and are ignited within the combustion chamber 902.The fuel injector head assembly is configured as outlined in thedetailed descriptions of FIG. 1 and FIG. 2. The sintered and/ormicro-fluidic flame barrier 900 is sealed 908 directly to the combustionchamber walls 910. Depending on application and material choice, seal908 can be made by braze joint, weld joint, mechanical interference fit,or bonded joint. An additional seal 912 is created at the junction ofthe interface sheath 904 and the thruster body cap 906. Depending on theapplication of the fuel injector head and material choice, seal 912 canbe made by braze joint, weld joint, mechanical interference fit, orbonded joint. One implementation used in testing prototypes of fuelinfector heads of the claimed invention successfully employed amechanical interference for the outer flame barrier seal 908, and abrazed interference sheath/thruster body seal 912.

FIG. 10 illustrates an isometric view of a regenerative cooled rocketthruster. Combustion reactants enter through the propellant inlet tube1000, pass through the fuel injector head as described in FIGS. 1 and 8,are ignited via a spark pulse delivered to the BNC connector 1002, andexit through an exit cone 1004. Other possible configurations for thecombustion chamber include, but are not limited to, refractory metalcombustion chambers, regeneratively cooled chambers, ceramic chambers,or any combination thereof.

For purposes of helping define the temperature extremes that a flamebarrier and its bonded joints must endure, FIG. 11 illustrates exemplarythermal analysis of the regeneratively cooled engine (FIGS. 9 and 10).In this case, the temperature of the uncombusted propellant is analyzedfrom the injection into a combustion chamber cooling jacket to the pointwhere the flame barrier is attached to the combustion liner wall 908. Anengine with a high temperature liner (FIG. 8) has a flame barriertemperature that has been previously analyzed in FIG. 5. The maximumfilter temperature of the regeneratively cooled engine is approximatelythe sum of the max jacket preheated propellant temperature shown in FIG.11 and the maximum temperature modeled in FIG. 5. In the exemplaryanalysis for the regeneratively cooled engine concept, the maximum flamebarrier temperature would, therefore, be <600° C. for a flame-front thatresides >1 micron from the flame barrier chamber surface.

Propellant injector head design must consider many factors, such as, butnot limited to, flame quenching distances, pressure drop variation dueto propellant heating in the flame barrier, mechanical loading on a hotporous structure (e.g., pressure loads on the heated injector face),loss of mechanical strength due to heating, possible sintering ofmicro-fluidic passageways and pores where the propellant injectionspeeds into the chamber are low enough to allow the flamefront tostabilize too close to the flame barrier surface (see FIG. 4 and FIG.6). Furthermore, fuel injector head design must also factor in thematerial selection and fabrication steps necessary for providing hightemperature reliable bonds at the locations described infra. To verifythat high temperature bonding processes would not significantly alter orcause a sintered and/or micro-fluidic flame barrier to fail, a series ofexperiments were performed on sintered metal filters with various poresizes.

FIG. 12 illustrates experimental data of sintered metal filters exposedto oven heating to temperatures that may be encountered in actualoperation or during high temperature bonding processes. In thisexperiment a sintered metal filter's pressure drop versus mass flow ratewas measured before and after a filter had been heated to determine ifthere was any significant changes in the micro-fluidic structure basedon global pressure drop estimate properties. Oven heating temperaturescases of 500° C., 750° C. and 1000° C. were tested. As can be seen, verylittle permanent changes occurred to the filter. Furthermore, thesetemperatures are significantly higher than the internal filtertemperatures estimated previously using the theoretical analysis(described above) for the specific case where the flame-front can becontrolled to be >1 micron from the flame-barrier surface.

In some combustion or chemical reaction chamber scenarios, chamberpressures can potentially be quite high (e.g., 100's to >1000 psia).Furthermore, high mass flow rates and pulsed combustor operation cancause large pressure gradients to exist across an injector head. If theinjector head does not have sufficient mechanical strength, the porousstructure may open under tensile loading and a subsequent failureresulting in a flashback can occur. For this reason it is important toensure that the worst-case pressure loading in operation can not causean injector head mechanical failure. A flame barrier's resistance topressure loading can be estimated by measuring the tensile stresses thatfilter materials can endure prior to failure and modulus of elasticityof the material (measure of deflection of material under an appliedload).

FIG. 13 demonstrates tensile test data for a sintered metal flamebarrier. The sintered metal, in this case nickel 200, failed at ˜12500psi. Compared to the published base metal's tensile strength of 67000psi, a lower tensile strength of roughly 5.4 times is observed. Thislower tensile strength of the sintered metal must be accommodated withgreater flame barrier thickness than would normally be required with apure metal such as nickel 200. The slope of this curve is the modulus ofelasticity.

FIG. 14 illustrates the use of the designs shown in FIGS. 9 and 10 in anactual monopropellant engine. Long duration pulses were run to verifythat there is no variation in the flame barrier pressure dropcharacteristics as the result of exposure to high combustion chambertemperatures and pressures. Forensic analysis of the engine fuelinjector head after testing by machining the engine down into across-sectional view as shown in FIG. 9 indicated no observable thermalalteration of the flame barrier or spark ignition mechanism.

The present specification provides a complete description ofcompositions of matter, methodologies, systems and/or structures anduses in example implementations of the presently-described technology.Although various implementations of this technology have been describedabove with a certain degree of particularity, or with reference to oneor more individual implementations, those skilled in the art could makenumerous alterations to the disclosed implementations without departingfrom the spirit or scope of the technology hereof. Since manyimplementations can be made without departing from the spirit and scopeof the presently described technology, the appropriate scope resides inthe claims. Other implementations are therefore contemplated.Furthermore, it should be understood that any operations may beperformed in any order, unless explicitly claimed otherwise or aspecific order is inherently necessitated by the claim language. It isintended that all matter contained in the above description and shown inthe accompanying drawings shall be interpreted as illustrative only ofparticular implementations and are not limiting to the embodimentsshown. Changes in detail or structure may be made without departing fromthe basic elements of the present technology as defined in the followingclaims. In the claims of any corresponding utility application, unlessthe term “means” is used, none of the features or elements recitedtherein should be construed as means-plus-function limitations pursuantto 35 U.S.C. §112, ¶6.

1. A combustion system comprising: a housing defining a cooling chamberand a combustion chamber separated by a flame barrier, wherein thecooling chamber is disposed around an electrode assembly, and the flamebarrier comprises fluid paths with a diameter of less than about 10microns, the electrode assembly comprises an interface sheathencompassing an insulating tube which encompasses an electrode; and afuel inlet tube is disposed through the housing into the coolingchamber.
 2. The combustion system of claim 1, wherein the electrodeassembly is part of a spark ignition assembly that further comprises anelectrical connector and a power supply.
 3. The combustion system ofclaim 1, wherein the combustion chamber comprises an ablative lineradjacent the housing.
 4. The combustion system of claim 1, wherein thecombustion chamber defines proximal and distal ends, wherein the flamebarrier is at the proximal and a throat constriction is at the distalend.
 5. The combustion system of claim 1, wherein the flame barriercontains fluid paths with a diameter of less than about 0.5 micron. 6.The combustion system of claim 1, wherein the electrode comprises a tip,single point, double point, triple point, quadruple point, star or splitconfiguration.
 7. The combustion system of claim 1, further comprising aseal between the flash barrier, the cooling chamber and the housing. 8.The combustion system of claim 1, wherein the cooling chamber receivesfuel via the inlet tube.
 9. A combustion system comprising: a housingdefining a chamber having distal and proximal ends; the housing defininga cooling chamber at the proximal end, a combustion chamber at thedistal end and a flame barrier between the cooling chamber and thecombustion chamber; an electrode assembly disposed through the proximalend of the housing through the cooling chamber and through the flamebarrier terminating at a surface of the flame barrier adjacent thecombustion chamber, wherein the electrode assembly comprises anelectrode disposed within an insulating tube, and wherein the insulatingtube is disposed within an interface sheath; and a fuel inlet tubedisposed through a side of the housing into the cooling chamber.
 10. Thecombustion system of claim 9, wherein the flame barrier comprises fluidpaths having a diameter of less than about 1 micron.
 11. The combustionsystem of claim 10, wherein the flame barrier comprises fluid pathshaving a diameter of less than about 0.5 microns.
 12. The combustionsystem of claim 11, wherein the flame barrier comprises fluid pathshaving a diameter of less than about 0.2 microns.
 13. The combustionsystem of claim 11 wherein the flame barrier comprises fluid pathshaving a diameter of less than about 1 cm.
 14. The combustion system ofclaim 9, wherein the interface sheath and the flame barrier comprisematerials having similar coefficients of thermal expansion.
 15. Thecombustion system of claim 14, wherein the interface sheath and theflame barrier comprise stainless steel alloy, pure nickel, nickelalloys, niobium, rhenium, molybdenum, tungsten, tantalum, tantalumalloys, sintered ceramic or laminate structures.
 16. The combustionsystem of claim 9, wherein the combustion chamber comprises an ablativeor high temperature liner adjacent the housing.
 17. The combustionsystem of claim 9, wherein the combustion chamber defines a throatconstriction at the distal end of the housing.
 18. The combustion systemof claim 9, wherein the electrode comprises a tip, single point, doublepoint, triple point, quadruple point, star or split configuration. 19.The combustion system of claim 9, further comprising a seal between theflash barrier, the cooling chamber and the housing.
 20. The combustionsystem of claim 9, wherein the cooling chamber receives fuel via theinlet tube.
 21. A method of preventing flashback between a combustionchamber and a feed propellant and for providing regenerative cooling ofan electrode assembly comprising: providing a propellant inlet into thecooling chamber, wherein a cooling chamber circumscribes the electrodeassembly; providing a micro-fluidic flame barrier to separate thecooling chamber and a combustion chamber, wherein the micro-fluidicflame barrier comprises fluid paths having a diameter of about 5 micronsor less, and running feed propellant through the fuel inlet, into thecooling chamber and through the flame barrier.
 22. The method of claim21, wherein the flame barrier comprises fluid paths having a diameter ofabout 2 microns or less.
 23. The method of claim 22, wherein the flamebarrier comprises fluid paths having a diameter of about 1 micron orless.
 24. The method of claim 23, wherein the flame barrier comprisesfluid paths having a diameter of about 0.5 microns or less.
 25. Themethod of claim 24, wherein the flame barrier comprises fluid pathshaving a diameter of about 0.2 microns or less.
 26. The method of claim21, wherein the flame barrier comprises fluid paths having a diameter ofless than about 1 cm.